For photodetector modules, technology has been developed for achieving impedance matching with a substrate to which the photodetector module is connected. An example of such technology is described in Japanese Unexamined Patent Application, Laid-Open No. H11-238916 (hereinafter referred to as Patent Document 1).
In order to prevent degradation of a high-frequency response characteristic due to an inductance component of a lead pin (leading pin) for connecting such a module to a substrate, the Patent Document 1 proposes to remove the inductance component by placing a metal sleeve on the periphery of the lead pin to form a coaxial line consisting of the lead pin and the metal sleeve.